Genetic Mechanism Of Deep-buried High-quality Dolostone Reservoir In Triassic Feixianguan Foramtion, Puguang Gasfield

As one of the biggest marine carbonate gas-fields, Puguang Gasfield is famous for its deep burial, large scale reserves, high production capacity and high content of H2S. However, the genesis of Feixianguan reservoir is not very clear yet. This paper focuses on formation mechanism of deep-buried Feixianguan dolostone reservoir, on the basis of a lot of research information at home and abroad, in carbonate sedimentology, reservoir geology and petroleum geology subject theories and methods as a guide. Through the research methods such as drilling, logging, coring, casting thin-sections, porosity and permeability analysis, mercury, scanning electron microscopy. X-rays, cathode luminescence, isotopes, trace elements and other testing analysis, reservoir characteristics of the Feixianguan Formation, and its sedimentary features, dolomitization, penecontemporaneous karstification, burial dissolutions and their influences on the reservoir had been discussed.In this paper, cores, casting thin-sections, physical property, mercury, logging interpretation and test results have been analysed to study the characteristics of Feixianguan reservoir, which is porosity reservoir, and mainly includes ?-and ?-type reservoir. The lithology of Feixianguan reservior is composed of residual oolitic dolostone, oolitic dolostone, coarse crystalline and medium-fine crystalline dolostones. Based on drilling, coring, logging and outcrop analysis, combined with tectonic evolution background and previous studies, sedimentary characteristics and facies type of Feixianguan Formation have been discussed. In the early-medium period of Feixianguan stage, the study area developed "two kinds of platform", "three kinds of basin" and oolitic banks around the platform margin. The oolitic bank is the most favorable facies, and it is the foundation of the reservoir. Primary intergranular pores are the bases of the oolitic dolostone reservoir. Through the Petrology and geochemistry analysis of dolomitized products, combined with vertical evolution and plane distribution of dotostones, seepage reflux dolomitization would be responsible to the Member 1 and Member 2 of Feixianguan Formation. This kind of dolomitization is the key to the dolostone reservoir, however, it did not create many new pores because its replacement with same volume. Combined with vertical and lateral distribution characteristics of karst zone, macro, micro and geochemical characteristics have been anlysed, and penecontemporaneous karstification model can be established. It indicated that in the end of Member 1 and Member 2 ofFeixianguan Formation deposited, affected by tectonic uplift and relative fall of sea-level eustacy, the oolitic dolostone had been suffered a period of karstification, which has two aspects of positive and reverse on the dolostone reservoir. Based on the analysis of burial dissolution characteristics and evidences, the source of acid fluid and dissolution mechanism during the burial environment had been discussed. Several questions have been further discussed, such as migration channel of acid fluid, material balance of burial dissolution, how burial dissolution contributes to the intergranular pore size. It reveals that during the burial, two episodes of dissolutions related to the thermal evolution of organic matter, were the key factors of deep buried dolostone reservoir.